A typical electronic image sensor comprises a number of photodiodes or other photosensitive elements arranged in a two-dimensional array. These elements are also commonly referred to as picture elements or “pixels” and the corresponding array is referred to as a pixel array. Light incident on the pixel array is converted to electrical charge by the photosensitive elements. Collected electrical charge for a given image capture period is read from the photosensitive elements of the pixel array using an active pixel sensor (APS) or charge-coupled device (CCD) arrangement.
As is well known, an image sensor may be implemented using complementary metal-oxide-semiconductor (CMOS) circuitry. An image sensor of this type is commonly referred to as a CMOS image sensor. In such an arrangement, each pixel comprises at least a photodiode and a transfer gate. The transfer gate is utilized to control the transfer of collected electrical charge from the photodiode to a sensing node in conjunction with an image readout process. The sensing node usually comprises a floating diffusion. Each pixel may include its own floating diffusion, or a single floating diffusion may be shared by a small group of pixels. As examples of the latter arrangement, groups of two, three or four pixels may each share a single floating diffusion. Each of the pixels of a given such group includes a transfer gate for controllably connecting the corresponding photodiode to the floating diffusion during image readout. Other readout circuitry may be shared between multiple pixels, such as a reset gate, an output transistor and a row select transistor.
A given transfer gate may be generally viewed as having two states of operation, namely, an on state, in which the transfer gate is conducting and transfers collected charge from the photodiode to the floating diffusion, and an off state, in which the transfer gate is non-conducting and effectively disconnects the photodiode from the floating diffusion. The transfer gate is placed in the on state or the off state by applying respective on state or off state voltage levels to the transfer gate. The transfer gate is usually placed in its on state for only brief periods of time sufficient to transfer collected charge from the photodiode and is otherwise kept in its off state. In accordance with conventional practice, the off state voltage level is usually set to a level that is sufficiently large to avoid leakage from the floating diffusion into the photodiode. This leakage is considered a form of photodiode dark current.
Another type of leakage can arise when using the above-described transfer gate. This leakage is referred to herein as gate-induced drain leakage (GIDL), and can cause an undesirable change in the floating diffusion potential between reset sampling of the floating diffusion prior to transfer of the collected charge from the photodiode and signal sampling of the floating diffusion subsequent to transfer of the collected charge from the photodiode. GIDL may be viewed as a form of floating diffusion dark current.
Conventional image readout techniques have not adequately addressed the relationship between photodiode dark current and floating diffusion dark current attributable to GIDL. Accordingly, a need exists for improved image readout techniques that provide an appropriate mechanism for reducing the negative impacts of both of these types of dark current on image sensor performance.